1. Field of the Invention
The present invention relates to a compact zoom lens, and in particular to improvements in a compact telephoto zoom lens which comprises, in succession from the object side, three positive, negative and positive groups.
2. Related Background Art
A telephoto zoom lens comprising, in succession from the object side, three groups having positive, negative and positive refractive powers is a developed form of the well-known telephoto zoom lens comprising four positive, negative, positive and positive groups. In a telephoto lens of the three-group construction of this type, the positive third group in the conventional four-group construction for keeping the position of the image surface constant and the positive fourth group which is an imaging group are made integral with each other to constitute a positive third lens group and further, the third lens group including the imaging group is moved along the optic axis with the other lens groups during zooming, whereby a magnification changing operation may be accomplished. Accordingly, the magnification changing operation is borne in common by the second lens group and the third lens group and therefore, the amount of magnification change by the negative second lens group which served as the conventional magnification changing group, that is, the variation in magnification caused by the positional relation between the image point of the first lens group and the second lens group during zooming, could be made smaller in this zoom lens of three-group construction than in the conventional telephoto zoom lens of four-group construction.
In the conventional zoom lens of three-group construction of this type, however, if the refractive power of the first lens group is strengthened to make the magnification changing system more compact, the imaging magnification of the second lens group will become great and the image point of the second lens group, namely, the object point of the third lens group, will move greatly toward the object side. Therefore, it becomes difficult to sufficiently secure the spacing between the second lens group and the third lens group with the focal length of the zoom lens being maximum at the telephoto end. Accordingly, to solve this, it becomes necessary that the third lens group be made into a telephoto type in which the telephoto ratio (the ratio of the full length to the focal length) is extremely small. However, making the third lens group which is the imaging group into a telephoto type in which the telephoto ratio is extremely small has led to the disadvantage that it becomes difficult to correct spherical aberration, particularly the spherical aberration at the telephoto side on which the focal length is long.
Further, if the refractive power of the second lens group having a negative refractive power is strengthened to provide a compact construction, the positive refractive power component in the second lens group will be relatively weakened and it will become difficult to secure the degree of freedom of the aberration correction of the lens group. Therefore, it will become difficult to correct the fluctuation of various aberrations caused by zooming. If in order to overcome this, an attempt is made to make the curvature of the joined surface of the negative lens and the positive lens in the second lens group sharper and strengthen the positive refractive power component, it has led to the disadvantage that fluctuation of chromatic spherical aberration is caused by zooming.